An ordered three-dimensional (3D) microstructure is an ordered 3D structure at the micrometer or nanometer scale. 3D microstructures can be manufactured from polymer materials such as polymer cellular materials. Currently, polymer cellular materials that are mass produced are created through various foaming processes, which all yield random (not ordered) 3D microstructures. 3D microstructures may also be known as “micro-trusses.”
A stereolithography technique provides a method to build a 3D microstructure in a layer-by-layer process. This process usually involves a platform that is lowered into a photo-monomer bath in discrete steps. At each layer, a laser is used to scan over the area of the photo-monomer that is to be cured (i.e., polymerized) for that particular layer. Once the layer is cured, the platform is lowered, and the process is repeated until the complete 3D structure is created. Modifications to the stereolithography technique have been developed to improve the resolution by using laser optics and special resin formulations. Modifications have also been developed to decrease the fabrication time of the 3D structure by using a dynamic pattern generator to cure an entire layer at once.
3D ordered polymer cellular structures have also been created using optical interference pattern techniques, also called holographic lithography; however, structures made using these techniques have an ordered structure at the nanometer scale and the structures are limited to the possible interference patterns.
A polymer optical waveguide can be formed in certain photopolymers that undergo a refractive index change during the polymerization process. When a monomer that is photo-sensitive is exposed to light (e.g., UV light) under the right conditions, the initial area of polymerization, such as a small circular area, will “trap” the light and guide it to the tip of the polymerized region due to the index of refraction change, further advancing that polymerized region. This process will continue, leading to the formation of a waveguide structure with approximately the same cross-sectional dimensions along its entire length. Prior techniques to create polymer optical waveguides have only allowed one or a few waveguides to be formed.
The phenomenon of photopolymerizable resins and their use to create self-propagating polymer waveguides with a collimated beam of light is known and multiple examples are provided, for example, in Kewitch and Yariv, “Nonlinear optical properties for projection photolithography,” Appl Phys Lett. 68(4) 22 (1996); Kagami et al., “Light induced three dimensional optical waveguide,” Appl Phys Lett. 79(8) 1079 (2001); Shoji and Kawata, “Optically induced growth of fiber patterns in a photopolymerizable resin,” Appl Phys Lett. 75(5) 737 (1999); and Yamashita et al., “Fabrication of self-written waveguide in photosensitive polyimide resin by controlling photochemical reaction of photosensitizer,” Appl Phys Lett. 85(18) 3962 (2004), which are each incorporated by reference herein.
Previous versions of self-propagating polymer optical waveguide systems disclosed monomer formulations based on thiol-ene polymerization, as set forth in U.S. Pat. No. 8,017,193 issued Sep. 13, 2011 to Zhou and Jacobsen at HRL Laboratories, LLC in Malibu, Calif., United States. U.S. Pat. No. 8,017,193 is hereby incorporated by reference herein in its entirety for all purposes. This patent describes formation of a polymeric micro-truss structure using monomer formulations appropriate for a thiol-ene system. This system produces high molecular weight by an alternating reaction between a thyil radical reacting with a terminal unsaturated group followed by the reaction of a hydrogen radical with the carbon-centered radical to regenerate a thyil radical and begin the process again.
While capable of producing a variety of polymeric structures, the available range of monomers along with structural variations and chemical functionality available to such a polymerization system is limited. In particular, the system disclosed in U.S. Pat. No. 8,017,193 is relatively insensitive towards oxygen while growing the network, reducing the need for resin purification and processing. Also, a low amount of heat is evolved upon growth of the network, preventing thermal decomposition of photoinitiator and runaway polymerization outside the regions directly illuminated with UV light. However, the need for low heat release restricts production of micro-truss systems to be limited largely to commercially available species containing multifunctional thiol and unsaturated moieties.
See Hoyle and Bowman, “Thiol-ene click chemistry,” Angew Chem Int Ed. 49 1540 (2010) for an overview of the advantages, limitations, and applications of thiol-ene chemistry; and Cramer et al., “Thiol-ene polymerization mechanism and rate limiting step changes for various vinyl functional group chemistries,” Macromolecules 36 7964-7969 (2003) for a discussion of thiol-ene kinetics.
What are especially needed are improved monomer formulations and methods capable of producing microstructures. These methods ideally would enable micro-truss fabrication with formulations containing unsaturated species as the exclusive reactive group (i.e., without thiol), without runaway polymerizations or inhibition of growing networks from oxygen. It is desired to broaden the range of available monomer system available to the micro-truss. The availability and range of chemical functionality of pure unsaturated polymerization systems is much greater, thus potentially broadening the synthetic choices available to the micro-truss developer.
Such advances could greatly improve the functionality and mechanical properties available to micro-truss structures. In addition, the cost of monomer resins can be decreased due to incorporation of inexpensive monomers used in commodity plastics (e.g., methyl methacrylate) produced industrially in high volume.